This invention relates to a series ballast circuit for lamps having a ballast transformer for limiting the open circuit voltage and the voltage-ampere load of the lamp, and is particularly useful for series ballast circuits for sodium vapor lamps connected to a constant current series regulator loop.
Series ballast circuits have been and are still being used for various lighting applications such as street lighting and airport runway lighting. Series ballast circuits cause the same current to flow through all of the circuits of the lamps and are primarily used to regulate the power to the lamps. Although such circuits were originally used to operate incandescent lamps, many have been converted or are in the process of conversion to operate with gas discharge lamps most commonly of high pressure sodium (HPS) vapor lamps types. In a converted circuit, the voltage for initiating and maintaining the discharge of the HPS vapor lamp is derived from a ballast circuit typically connected to a constant current power source. In typical installations, the ballast circuits respectively related to the number of lamps involved are connected in a series loop across a secondary coil of a constant current regulator or power transformer.
In HPS vapor lamps a relatively high potential is necessary in order to initiate the arc or ionization condition for the lamp itself. This high potential is derived from the ballast transformer located at the input of the ballast circuit and is connected across the constant current power source. Because the secondary of the ballast transformer looks at or sees a very high impedance load until the arc condition is established in the lamp, the voltage across the primary and secondary of the ballast transformer is initially quite high. At this high voltage condition and in consideration of the constant current being supplied to the ballast transformer, the volt-ampere input required for each ballast circuit is therefore at a peak during the initial starting or extinguished condition of the lamp.
In such series ballasting arrangements connected to a constant current source, it is necessary to have only a certain maximum voltage dropped across the ballast circuits so as to be able to regulate the related current. This limitation is accomplished by limiting the number of ballast circuits that are interconnected to the constant current source. For a constant current source having a rating of 10 KW which is interconnected to ballast circuit for operating typical 200 W HPS vapor lamps having a volt-ampere requirement of 750 (VA), a limitation of (10 KW/750 VA) thirteen (13) ballast circuits is employed.
HPS vapor lamp ballast circuits which reduce the volt-ampere load seen by a constant current source and thereby increase the allowable ballast circuits interconnected to constant current sources are described in U.S. Pat. Nos. 4,339,695 and 4,441,056. Such HPS vapor lamp ballast circuits include a voltage limiting or clamping circuit arranged across a secondary of the ballast transformer that are operated in such a manner so as to reduce the volt-ampere requirement of the ballast circuits. The voltage limiting circuits employ electronic devices which when subjected to severe operating conditions, typically experienced during street lighting applications, encounter relatively large surge currents which commonly cause failures of such devices. It is desired that means more reliable than electronic devices which are susceptible to surge current failures be provided to reduce the volt-ampere requirement of the ballast circuits.
Accordingly, it is an object of the present invention to provide series ballast circuits having a non-electronic means to provide a high desired reliability, while at the same time reducing the volt-ampere requirement of the ballast circuit itself and allowing for an increase in the number of ballast circuits that may be interconnected to a constant current power source.